1. Field of the Invention
The invention relates to a four-wheel drive vehicle that enables switching between a two-wheel driving state and a four-wheel driving state.
2. Description of the Related Art
Four-wheel drive vehicles have been available which include a lateral pair of main driving wheels and a lateral pair of auxiliary driving wheels and in which a driving force of a driving source is constantly transmitted to the main driving wheels, whereas the driving force of the driving source is transmitted to the auxiliary driving wheels only during four-wheel driving. In some of the four-wheel drive vehicles, the auxiliary driving wheels are driven by a driving force (torque) transmitted via a friction clutch that is hydraulically pressed (see, for example, Japanese Patent Application Publication No. 2014-231858 (JP 2014-231858 A)).
A four-wheel drive vehicle described in JP 2014-231858 A includes a driving force distribution apparatus that distributes the driving force between the right and left auxiliary driving wheels while permitting differential motion. The driving force distribution apparatus includes a pump that discharges hydraulic oil, a control valve having an opening varied according to the amperage of a supplied current, a piston housed in a cylinder to which the hydraulic oil with the pressure thereof adjusted by the control valve is supplied, and the friction clutch pressed by the piston. In the friction clutch, a plurality of clutch plates are brought into frictional contact with one another by a pressing force applied by the piston, to transmit a torque corresponding to the pressing force.
A four-wheel driving vehicle described in JP 2014-231858 A includes a jaw clutch (positive clutch) that enables interruption of transmission of the driving force from the driving source to a propeller shaft that transmits the driving force in a front-rear direction of the vehicle. During two-wheel driving, the transmission of the driving force via the jaw clutch is interrupted, and the friction clutch of the driving force distribution apparatus is disengaged. Consequently, even during traveling, the propeller shaft stops rotating to reduce resistance to stirring of lubricant in gear mechanisms provided at opposite ends of the propeller shaft.
When the four-wheel drive vehicle configured as described above shifts from the two-wheel driving state to the four-wheel driving state, the friction clutch of the driving force distribution apparatus is pressed by a relatively weak pressing force to transmit a torque from the auxiliary driving wheels to the propeller shaft via the friction clutch, rotating the propeller shaft, and rotational synchronization is performed at the jaw clutch, which is then engaged. When the four-wheel drive vehicle shifts from the four-wheel driving state to the two-wheel driving state, the friction clutch of the driving force distribution apparatus is pressed by a relatively weak pressing force to set the jaw clutch to an interruption state with vibration of the propeller shaft suppressed. Subsequently, the friction clutch is disengaged.
In this case, when the pressing force applied to the friction clutch is not precisely controlled, the propeller shaft fails to be appropriately accelerated when the two-wheel driving state is switched to the four-wheel driving state or vibration of the propeller shaft fails to be appropriately suppressed when the four-wheel driving state is switched to the two-wheel driving state. Thus, switching of the driving state may take a long time or possible vibration or noise may make an occupant uneasy and uncomfortable. Therefore, a four-wheel driving vehicle has been demanded which enables the torque transmitted via the friction clutch for transmitting the driving force to the auxiliary driving wheels to be precisely controlled particularly in a low torque region.